Radar wind profiler, also referred to as wind profiler radar, wind profiling radar, atmospheric radar, or clear-air Doppler radar (hereafter abbreviated to WPR) is an instrument that measures height profiles of wind velocity in clear air. WPR detects echoes produced by perturbations of the radio refractive index with a scale half of the radar wavelength (i.e., Bragg scale). The mechanism of radio wave scattering in clear air was theoretically and experimentally understood in the 1960s. Since the 1970s, large-sized Doppler radars for observing wind and turbulence in the mesosphere, stratosphere, and the troposphere (MST radars) have been developed. Owing to their capability of measuring wind and turbulence with excellent time and height resolution, they have made great contributions to describing and clarifying the dynamical processes in the atmosphere.